The application generally relates to optical elements, and in particular, materials and configuration for optical assemblies and telescopes.
Refractive optics assemblies adapted for both the visible (approximately 400-700 nm) and the mid-wavelength infrared (MWIR) (approximately 3-5 μm) portions of the electromagnetic spectrum are known.
FIG. 1, for example, shows a raytrace schematic of a conventional optical assembly. Conventional optical assembly 100 may be configured as a wide-field of view (WFOV) Galilean afocal telescope as shown. One such attachment is used in the a helicopter targeting turret system.
Conventional optical assembly 100 generally includes first lens 110, second lens 120, third lens 130, fourth lens 140, fifth lens 150, sixth lens 160 and seventh lens 170. First through fourth lenses 110-140 may be arranged to form an objective, while fifth to seventh lenses 150-170 may be arranged to form an eyepiece. A housing (not shown) is provided to support the various lens elements. Optical assembly 100 is approximately 4.0 inches in length and 1.0 inch in diameter.
In use, light rays 101 in space 105 pass through first lens 110, space 115, second lens 120, space 125, third lens 130, space 135, fourth lens 140, space 145, fifth lens 150, space 155, sixth lens 160, space 165, and seventh lens 170 to space 175.
Using optical assembly 100 causes the focal plane array (FPA) to view more field of view (FOV), but through a smaller aperture. The afocal telescope does not have an intermediate image (or final image, for that matter). Rather, collimated light enters the telescope and collimated light exits the telescope. The telescope is Galilean in that there is no intermediate image, as opposed to Newtonian telescopes which have intermediate images. Newtonian telescopes have real exit pupils while Galilean telescopes do not have real exit pupils.
Table 1 shows one optical prescription for conventional optical assembly 100. The afocal magnification is approximately 4×, by taking an aperture of 0.618 inches and FOV of 9.4 degrees in object space, and converting it into an aperture of 0.153 inches (4× smaller) and FOV of 38.14 degrees (4× larger). Conventional optical assembly 100 is designed in the direction shown in the table, but is actually used in the reverse, where the smaller aperture and wider FOV faces object space, while the larger aperture and smaller FOV is on the FPA side of the optics path. Between this afocal attachment and the FPA there may be imager optics (not shown).
TABLE 1PRESCRIPTION FOR CONVENTIONAL VISIBLE AND MWIR AFOCAL TELESCOPESurfaceRadiusThicknessMaterialCCAdAeAfAg105Inf2.300air1101.2940.220CaF2115−7.329 0.020air1203.0180.130CaF2125−26.161 0.177air130−56.896 0.100ClZnS13523.696 0.041air1.022E−02−9.601E−030.14375−0.29522140−3.679 0.100Al2031455.4682.226air150−0.515 0.100BaF2−1.248971551 0900 018air1601.0830.125ClZnS−0.851566.024E−020.34278−0.26262−0.106521651.2260.019air1701.1990.125Al203−1.983421752.4590.500AirEntrance aperture: 0.660 diameterEntrance FOV: 9.4 deg totalSpectral bands: 0.45-0.75 um, and 3.7-4.8 umAd, Ae, Af, Ag are aspheric constantsCC is a conic constant
As noted in Table 1, optical assembly 100 is comprised of four materials: barium fluoride (BaF2), clear transient ClearTran zinc sulfide (ClZnS), calcium fluoride (CaF2), and aluminum oxide (Al2O3). In this assembly, Al2O3 is used as a crown and CaF2 is used as a flint. At visible wavelengths, the ClZnS lens functions as a flint, and the CaF2, BaF2, and Al2O3 lenses function as crowns. However, at MWIR wavelengths, the Al2O3 lens functions as the flint, and the CaF2, BaF2, and ClZnS lenses function as crowns. The term crown generally refers to a low dispersion material, while the term flint generally refers to a high dispersion material.
Optical assembly 100 is capable of transmitting wavelength from 0.48 μm to 4.8 μm, or about 10 octaves. However, this provides transmission in the visible and the MWIR spectra only. The optical system has been optimized to provide best image quality from about 0.45 to 0.75 microns and from 3.7 to 4.8 microns.
In addition, optics assemblies that transmit both the MWIR and long-wavelength infrared (LWIR) (approximately 8-12 μm) portions of the spectrum are known. With the advent of 2-color (MWIR and LWIR) detector arrays, it is desired that certain optical assemblies also cover the visible and both the MWIR and the LWIR infrared bands.
However, an optical assembly for achieving transmission in the visible, MWIR and LWIR portions of the electromagnetic spectrum has not yet been realized.